Laser characterization system and process

ABSTRACT

A system and process for automatically characterizing a plurality of external cavity semiconductor laser chips on a semiconductor laser bar separated from a semiconductor wafer. The system includes a diffraction grating and a steering mirror mounted on a rotary stage for rotating the diffraction grating through a range of diffraction angles. A laser bar positioning stage for automatically aligning each laser chip in a laser bar with the diffraction grating. Reflecting a laser beam emitted from a laser chip in a laser bar with diffraction grating and steering mirror to the laser analyzer. Automatically rotating the diffraction grating through a range of diffraction angles relative to the laser beam and automatically characterizing the laser optical properties such as spectra, power, or spatial modes with the laser analyzer at each diffraction angle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 61/419,042 filed on Dec. 2, 2010,the content of which is relied upon and incorporated herein by referencein its entirety.

BACKGROUND

The disclosure relates to a system and process for characterization ofsemiconductor lasers, and more particularly for characterization ofexternal cavity semiconductor lasers, and more particularly forautomated testing and characterization of each external cavitysemiconductor laser chips in a bar of laser chips prior to separationand mounting of each individual chip from the bar.

A quantum cascade laser (QCL) is a type of unipolar semiconductor laserthat emits light primarily in mid-infrared (MIR) and far-infrared (FIR)wavelength range, e.g. in a range of wavelengths from about 3 μm toabout 15 μm. An external cavity quantum cascade laser (EC-QCL) is alaser system combining a quantum cascade gain block (e.g. a laser chipwith antireflection coatings on one facet) and an external cavity. Theexternal cavity typically includes a collimation lens and a diffractiongrating mirror (or simply diffraction grating). In a common Littrowconfiguration for a tunable external cavity semiconductor diode laser,the light emitted from the quantum cascade gain block is reflected bythe diffraction grating at the first order diffraction back along theoriginal beam path and back into the quantum cascade gain block toachieve lasing. Such a laser system is typically carefully designed sothat it can lase at a single wavelength, which is determined by thegrating angle (or Littrow angle) of the grating mirror. The gratingangle is the angle between an axis extending normal to the diffractiongrating and the axis of the path of the beam of light emitted from thequantum cascade gain block. When the diffraction grating rotates orpivots, the grating angle changes, and the lasing wavelength of thelaser beam produced by the EC-QCL also changes. The lasing wavelength ofan EC-QCL laser system can therefore be tuned within a certain range byrotating the grating mirror. The range within which the lasingwavelength can be tuned is determined by parameters such as the gainprofile of the quantum cascade gain block, the anti-reflective (AR)coating on the facet of the gain block, the coupling efficiency betweenthe gain block and external cavity, and the reflectivity of the firstorder diffraction of the grating mirror.

EC-QCLs provide a relatively wide spectral tunability of a singlefrequency mid-IR radiation, which finds numerous applications in mid-IRspectroscopy and molecular sensing. Characterization and optimization ofthe EC-QCL gain media parameters, such as characterization of the tuningrange and electrical properties of the laser, play one of the mostimportant roles in the EC-QCL development and design. Traditionalmethods for fabrication, characterization and selection of QCL gainmedia in the EC-QCL configuration requires several stages: 1) design,growth, and processing of the wafers, 2) wafer cleaving and separationinto single EC-QCLs chips, 3) chip die bonding/mounting, 4) chip facetcoating (one at a time), 5) individual EC-QCL chip testing,characterization and optimum gain media selection. While stage #1 isusually optimized to the best industrial standards, the processes ofstage #2 to stage #5 are typically performed manually and are difficultto implement in high-volume industrial manufacturing and research anddevelopment environments. The separating, mounting, testing andcharacterization of each individual EC-QCL chip requires a significantamount of time and a well qualified operator to perform the complexlaser-cavity alignment. Characterization of the laser chips is thereforea costly, time consuming and labor intensive process. There is a needfor a process and system for testing and characterizing EC-QCL and othertypes of external cavity (EC) laser chips at the earliest stage offabrications, such as prior to one or more of stage #2 through stage #5described above, that eliminates wasted time and effort spent in stages#2 to stage #5 and provides a more efficient and cost-effective lasertesting and characterization process.

SUMMARY

One embodiment as described herein provides a system and process forperforming fully automated testing and characterization ofanti-reflective (AR) and/or high reflection (HR) coated external cavitylaser bars (with 40 QC gain blocks) before separating them intoindividual laser chips/diodes. The system and process hereof assures astable optical axis of the output laser beam with no beam steering andno walk-off during the tuning process. The system and process hereofalso provides a stable external cavity design with precise, automaticlaser-cavity alignment and/or characterization. By providing access toonly one facet of quantum cascade laser chip at a time, complicationswith the alignment of a second collimating lens for the output laserradiation is avoided, which is particularly beneficial in the case ofquantum cascade lasers having waveguides of different lengths.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theembodiments as described herein, including the detailed descriptionwhich follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understanding the natureand character of the claims. The accompanying drawings are included toprovide a further understanding, and are incorporated in and constitutea part of this specification. The drawings illustrate one or moreembodiment(s), and together with the description serve to explainprinciples and operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective schematic illustration of a bar of QCL laserchips cut from a wafer with rows of QCL chips formed thereon;

FIG. 2 a schematic illustration of an embodiment of a laser bar testingand characterization system according to an embodiment hereof;

FIG. 3 is a functional block diagram of the laser bar testing andcharacterization system of FIG. 3;

FIG. 4 is a perspective schematic illustration of an embodiment of alaser bar vacuum chuck fixture for use in the system of FIGS. 2 and 3;

FIG. 5 is a schematic illustration of an embodiment of an externalcavity module (EC module) for use in the system of FIGS. 2 and 3;

FIG. 6 is a schematic illustration of the alignment of the diffractiongrating and steering mirror on a rotary stage in the EC module of FIG.5;

FIG. 7 is a schematic illustration of the alignment of the diffractiongrating with the rotational axis of the rotary stage in the EC module ofFIG. 5;

FIG. 8 is a schematic illustration of the alignment of the collimatinglens with the diffraction grating and rotary stage in the EC module ofFIG. 5;

FIG. 9 is a schematic illustration of the alignment of a QCL chip withthe collimating lens in the EC module of FIG. 5;

FIG. 10 is a schematic illustration of the alignment of the diffractiongrating at the glazing angle in the EC module of FIG. 5 with the beamfrom a laser chip in the system of FIGS. 2 and 3;

FIG. 11 is a plot showing EC spectral tunability and threshold currentspectra for an EC-QCL chip operating at 3.5 μm as characterized by thesystem of FIGS. 2 and 3;

FIG. 12 is a plot showing threshold current vs. grating angle for andselect EC-QCL chips on the same bar as characterized by the system ofFIGS. 2 and 3; and

FIG. 13 is a series of plots showing the tuning spectra of selectquantum cascade lasers on the same bar as characterized by the system ofFIGS. 2 and 3.

DETAILED DESCRIPTION

Reference will now be made in detail to the present preferredembodiment(s), an example of which is/are illustrated in theaccompanying drawings. Whenever possible, the same reference numeralswill be used throughout the drawings to refer to the same or like parts.

The present disclosure provides a system and process for automaticalignment, testing and characterization of a plurality of laser chips ona bar cut from a semiconductor wafer, before separating the laser chipsinto individual laser chips or mounting of the individual chips forsubsequent handling or testing and characterization. As schematicallyillustrated in FIG. 1, a semiconductor wafer 10 has a plurality of laserchips 3 formed thereon in a plurality of rows. As illustrated in FIG. 1,the wafer 10 is cleaved to separate individual rows of laser chips fromthe wafer, 1.e. to separate individual bars 1 of semiconductor materialhaving a plurality of laser chips 3 arrayed thereon (a “laser bars”)from the wafer. The laser chips may be, for example, 40 QCL Fabry-Perotcavity waveguide laser chips or gain blocks arrayed I parallel along thelength of the laser bars 1. The cleaved facet 5 on one side of a laserbar 1 may be provided with a highly-reflective (HR) coating on one endof the waveguide and the cleaved facet 7 on the other side of the laserbar may be provided with an (AR) coating on the other end of thewaveguide. Uncoated chips may also be tested with the system and processdescribed herein.

One embodiment of a system for automatic alignment, testing andcharacterization of semiconductor laser chips 3 on a laser bar 1 willnow be described with reference to FIGS. 2 and 3. A lasercharacterization system according to an embodiment hereof includes alaser bar positioning stage module (PS module) 100 and an externalcavity module (EC module) 200. The PS module 100 is configured to movethe laser bar 1 in a controlled manner relative to the external cavity(EC) module 200, in order to align the laser chips 3, one by one, withthe EC module. The EC module is in turn configured to rotate or pivot adiffraction grating 210 in a controlled manner in relation to the laserchips on the PS module. A collimating lens 212 is positioned between thePS module 200 and the EC module to collimate the light L emitted from alaser chip 3 aligned with the EC module and focus a collimated beam oflight Blonto the diffraction grating 210. The collimating lens may be apre-aligned part of the EC module 200. The collimating lens 212 may be afast (f-number f#≦1) aspheric lens with AR-coating, corrected toeliminate spherical aberrations, designed especially for calibration ofthe highly divergent beams. By way of example only, the collimating lens212 may be a 1″ diameter, f/0.6, Ge, AR-coated lens designed forwavelengths between 3 and 12 μm. The collimating lens may be mounted ontranslation stage or lens alignment module, such as a conventional 3Dtranslation stage as are known in the art for providing linear motion.Stage may be preferably equipped with remotely controlled piezo-motorsfor positioning lens 212 as needed for laser optical alignment.

The entire system in one embodiment hereof may be sealed within avacuum-tight housing (not shown), so the simplest way to align thecollimating lens 212 is with an automated movable stage 238 or otherlens positioning mechanism. In embodiments where the collimating lens isnot inside a housing, other modes of adjustment, including manualadjustment, may be used.

According to one embodiment hereof, the PS module 100 may include avacuum chuck 110, a positioning stage 120, and an electronic probe 130.The vacuum chuck 110 is mounted on top of the positioning stage 120 forprecisely mounting a laser bar 1 to be characterized on the positioningstage 120. As illustrated in FIG. 4, a row of vacuum holes 112 areprovided in the planar top surface 113 of the vacuum chuck and form alaser bar mounting station parallel to and adjacent an edge of thevacuum chuck facing the EC module 200. Air is drawn through the vacuumholes in order to create a suction on the bottom surface of a laser bar1 placed on the mounting station (e.g. on the top surface of the chuckover the vacuum holes) and thereby secure the laser bar in place on themounting station on the chuck. A pair of precisely located alignmentabutments 115, 116 and a precisely located locating abutment 117 extendup from the top surface of the vacuum chuck. The alignment abutments115, 116 engage opposing ends of the AR coated edge 7 of the laser bar 1for pre-aligning the laser bar 1 relative to the positioning stage 120and the EC module. The locating abutment 117 engages one end of thelaser bar to transversely pre-locate the laser bar on the chuck 110. Thealignment abutments are illustrated as plates, but it will beappreciated that the alignment abutments could be pins or otherstructure. Similarly, the locating abutment 116 is illustrated as a pin,but could be a plate or other structure. Alternatively, in place of thealignment and locating abutments, a precisely formed and positionedlocating slot may be precisely formed in the top surface of the chuckfor receiving, aligning and locating a laser bar on the positioningstage. In which case, the row of vacuum holes 112 would be provided inthe bottom of the locating slot. The vacuum chuck 110 may be formed of aconductive material, such as copper, and temperature controlled with athermoelectric cooler (TEC) module 150 in order to cool the laser bar 1and maintain the laser bar at a substantially constant temperature.Alternatively, cooled fluid from a cooling unit, such as a refrigerator,may be circulated through coolant passages provide in the chuck in placeof the TEC.

The positioning stage 120 is connected to a computer or controller 160for precisely controlling the motion of positioning stage andautomatically aligning individual laser chips 3 on the laser bar withthe EC module 200 for automated testing and characterization. Forexample, the positioning stage 120 may be a precision 6 axis-translationstage, for example a 6 axis hexapod, such as a PI F-206.S HexAlign™ fromPhysik Instrumente (PI) GmbH. The 6 axis-hexapod positioning stageprovides precise translation and rotation of a laser bar 1 forrepeatable locating and alignment of individual laser chips 3 on a laserbar 1 relative to the EC module 200.

The electronic probe 130 is mounted on the positioning stage 120 (or thechuck 110) by a motorized probe manipulator 132. The probe manipulator132 is controlled by the controller 160 for automated movement of theelectronic probe 130 into and out of top electrical contact with a laserchip 3 on the laser bar 1 that is aligned with the EC module to activatethe laser chip. The laser chip is electrically grounded through theconductive vacuum chuck 110. Alternatively, an electrical contact maybeprovided on the top surface 113 of the vacuum chuck in the mountingstation for contacting and grounding the laser chips on the laser bar.The electronic probe 130 may be mounted on the stage by one or moremotorized micro stages for automated movement of the probe into and outof contact with each individual laser chip on the laser bar. Forexample, the electronic probe may be mounted on 2 motorized microstages, one microstage for movement in the horizontal x direction alongthe laser bar from laser chip to laser chip and one micro stage formovement in the vertical z direction into and out of engagement with aselect laser chip. For example, the probe manipulator may include twoMM-3M motorized Micro-Mini™ stages from National Aperture, Inc. Theprobe manipulator is co-located on the positioning stage 120 with thevacuum chuck 110, so that the electronic probe 130 moves with the laserbar 1 on the vacuum chuck 110 as the positioning stage 120 is moved toalign a laser chip 3 with the EC module 200.

An external cavity (“EC”) module 200 according to one embodiment hereofis schematically shown in FIGS. 2, 3 and 5. The EC module 200 includes adiffraction grating 210 and a beam steering mirror 220 pre-aligned on arotary stage 230 to form a diffraction grating unit. The diffractiongrating 210 and the beam steering mirror 220 are mounted at a rightangle to each other on a vertically oriented rotary stage 230. Therotary stage 230 has a horizontal rotary axis 232 for rotating thediffraction grating 210 and the beam steering mirror 220 relative to alaser chip to be tested in a laser bar mounted on the vacuum chuck 110at the mounting station. The diffraction grating 210 and the beamsteering mirror 220 are mounted at a right angle to each other, so thatthe output beam B2 remains substantially steady in its trajectory anddoes not “walk-off” as the diffraction grating is rotated duringcharacterization.

The laser analyzer 140 is located to receive and analyze the output beamB2 and to profile the spatial distribution of the beam. The laseranalyzer 140 may be a Fourier Transfer Infrared spectrometer (FTIR) forspectral characterization of the laser chips or any other optical testequipment such as a grating spectrometer, M̂2 (beam quality) measurementsystem, simple power meter, polarimeter, an interferometer based on theneeds of the user. The optical signal detected by the analyzer can bedemodulated by a lock-in amplifier 170 for improved noise rejection.

The diffraction grating 210 may be any suitable reflective grating thathas appropriate resolving power and efficiency, including but notlimited to a ruled reflective diffraction gratings blazed for thedesired wavelength region. An example of a suitable grating is a ruleddiffraction grating blazed for a wavelength of 5.4 μm having 300grooves/mm. Likewise, the steering mirror 220 may comprise any suitablereflecting surface, including but not limited to gold, silver oraluminum coated mirrors, which can provide high reflectivity withinbroad range of wavelengths.

The rotary stage may be mounted on a support frame 234 and the supportframe may be vertically adjustably in the Z direction mounted on acolumn (not shown in FIG. 5) or other supporting structure. The rotarystage 230 may be mounted on the support frame 234 via tilt stage 240 foradjusting the horizontal tilt of the rotary stage 230, and thereforeadjusting the tilt of the diffraction grating 210 and the beam steeringmirror 220 relative to a laser chip to be tested mounted on the vacuumchuck 110. The pivotal axis of the tilt stage should be perpendicular tothe axis of rotation of the grating rotary stage 230. The collimatinglens may be mounted on an arm 236 extending from the support frame 234via a lens positioning mechanism 238, such that the collimating lens maybe precisely located between a laser chip 3 to be tested mounted on thevacuum chuck 110 and the diffraction grating 210. Suitable drivedevices, such as stepper motors, may be employed to rotate the rotarystage 232 and the tilt stage 240 in order to align the EC module, asdescribed in more detail hereinafter, and to move the support frame 234on the column under the automated control of the controller 160 or undermanual control.

There are many degrees of freedom of adjustment of the external cavityin external cavity lasers. In order to perform a reliable alignment ofthe EC module 200 with the laser chips 3 on a laser bar 1 and assuregood coupling of the EC module within the entire tuning range of thechips 3, the elements of the EC module should be carefully pre-aligned.In embodiments hereof where the collimating lens is part of the ECmodule, the following EC module pre-alignment requirements should besatisfied.

First EC module pre-alignment requirement. As illustrated in FIG. 7, thediffraction grating 210 should be mounted on the rotary stage 230 suchthat the grooves 242 in the diffraction grating 210 are aligned parallelto the rotary axis 232 of the rotary stage 230 (or the rotary axis 232).Otherwise, the reflected and diffracted beam will not coincide with thelaser optical axis when the diffraction grating is rotated.

Second EC module pre-alignment requirement. As illustrated in FIGS. 2and 6, the diffraction grating 210 and the beam steering mirror 220should be mounted on the rotary stage 230 such that the two planesdefined by the surfaces of the diffraction grating 210 and the steeringmirror 220 are oriented at a right angle to each other and theintersection of these two surfaces coincides with the rotary axis 232 ofthe rotary stage 230. Otherwise, the output beam B2 will “walk-off” whenthe diffraction grating is rotated during the wavelengthcharacterization process.

Third EC module pre-alignment requirement. As illustrated in FIG. 8, theorientation of the tilt stage 240 and/or the position of the collimatinglens 210 should be adjusted so that the rotary axis 232 is orientednormal to the axis 244 of the collimation lens (the lens axis 212).Otherwise, the diffraction beam reflected back toward laser chip 3 beingcharacterized will walk-off when the diffraction grating is rotatedduring the wavelength characterization process, which will reduce thecoupling efficiency of the EC with the laser chip 3

Once the EC module pre-alignment requirements are met, then thealignment of the elements of the EC module may be permanently fixed, sothat the EC module remains properly aligned for future lasercharacterizations.

If not already so mounted, the aligned EC module is now mounted on thecolumn or other supporting structure (not shown), such that the ECmodule is adjacent to and facing the PS module. In order for thereflected diffraction beam to be adequately couple back into the laserwaveguide/gain block of the laser chip 3 being characterized for lasingto be achieved, two EC module-PS module alignment requirements should bemet.

First EC module-PS module alignment requirement. As illustrated in FIG.9, the position and orientation of the positioning stage 120 and/or thevertical position of the EC module 200 on the column should be adjustedso that the axis of a light beam 248 emitted from a first laser chip 3to be characterized (or simply the beam axis 248) on a first end of alaser bar is aligned and coincides with the lens axis 244 of thecollimating lens.

Second EC module-PS module alignment requirement. As illustrated in FIG.10, the rotary stage should be adjusted/rotated so that surface of thediffraction grating 210 is oriented relative to the beam of light B1 atthe grating angle or Littrow angle Θ. The first and second EC module-PSmodule alignment requirements or steps may require manual pre-alignmentwhen the system is first initialized for a specific laser barconfiguration.

In order to characterize the chips 3 on a laser bar 1 in a processaccording to one embodiment hereof, the components of the EC module areapre-aligned and the EC module is pre-aligned with the PS module asdescribed above. The system is now ready to be auto-aligned prior tocharacterization the laser chips on a laser bar. In order to auto thesystem, first the electronic probe 130 is moved up and away from thealigning abutments 115, 116 and the locating abutment 117. A laser bar 1is then located on the mounting station on the top surface 113 of thevacuum chuck 110 with the AR coated face 7 of the laser bar in contactwith the alignment abutments 115, 116 and the end of the laser bar incontact with the locating abutment 117. The size and type of the laserbar is entered into the controller, which is pre-programmed toautomatically locate, align and analyze the laser chips on the laserbar. The laser bar 1 may, for example, be a bar of 40 EC-QCL laser chips(C₁, C₂, C₃ . . . C₄₀). It will be appreciated that the system andprocess hereof may be used to test and characterize other types oflasers. If the vacuum to the vacuum chuck is not already turned on, thevacuum is now turned on in order to retain the laser bar in place on thevacuum chuck. The desired range of grating angles Θ, i.e. the minimumand maximum grating angles and the number n of increments of gratingangles, Θ1, Θ₂, . . . Θ_(n), to be characterized are also entered intothe controller. The range of grating angles Θ to be tested is determinedby the width of the laser chip gain profile. For example, for a lasertuning range of from 4.5 um to 4.7 um, the grating angle range to betested can be calculated from the Bragg equation as approximately from 2degrees to 3 degrees. The number of increments n of grating angles Θ tobe tested may be chosen based on the maximum total characterization timeone can afford. For example, the number of increments n may be set to 15or 20.

The control software may be designed such that, before the actualcharacterization of each laser chip 3, an auto-alignment procedure isexecuted to find the optimum external cavity coupling position andalignment for each laser chip C₁, C₂, C₃ . . . C_(n). If not alreadydone during pre-alignment of the EC module and the PS module, thecontroller then rotates the rotary stage 230 to orient the diffractiongrating 210 at the grating angle Θ and translates and rotates thepositioning stage 120 to locate and align the first laser chip C₁opposite and in alignment with the diffraction grating 210. Thecontroller also moves the electronic probe 130 into contact with a firstlaser chip 3 C₁ on the laser bar 1. The controller then operates thefirst laser chip C₁ in a pulsed mode to activate the first laser chipC₁. The optical signal or output beam B2 is detected by a laser analyzer140. The laser analyzer may be used to profile the spatial distributionof the beam. The laser analyzer may be demodulated by a lock-inamplifier 170. The lock-in output of the spectrometer, which isproportional to the optical power of the output beam B2, is fed back tothe controller 160 for auto-alignment by automated movement of thepositioning stage 120 by the controller.

Final auto-alignment of the EC module with the PS module so that thefirst laser chip C₁ is precisely located and oriented in alignment withthe EC module may be performed as follows. The controller translates thepositioning stage 120 within a predetermined range of motion, forexample a translation of typically about +/−25 μm in the x and zdirections, for a total translational range of about 50 μm, while thepower of the emitted beam B2 is monitored by the laser analyzer 140. Theuser can elect to additionally perform auto-adjustment of the beamcollimation quality by translating the positioning stage 120 in the ydirection within a predetermined range of motion while simultaneouslyperforming x and z auto-alignment for each y position. This can, forexample, include a translation of typically about +/−10 μm in the ydirection. The controller records the location of the positioning stageat which the power of the emitted beam B2 by the first laser chip is thegreatest in memory as the aligned position for the first laser chip C1.

Once the first laser chip C₁ is optimally aligned, the system is readyto test and characterize the first laser chip. However, the system mayfirst optionally automatically determine pre-aligned positions for eachof the remaining laser chips C₂, C₃ . . . C. In order to determinepre-aligned positions for each of the remaining laser chips, thecontroller deactivates the electronic probe, retracts the electronicprobe from the laser bar, and moves the positioning stage to positionthe last laser chip C_(n) in pre-alignment with the EC module. Thecontroller then operates the last laser chip C_(n) in a pulsed mode andtranslates the positioning stage 120 as previously described toprecisely locate and align the last laser chip C_(n) with the EC moduleand store the optimally aligned position for the last laser chip C_(n)in the memory of the controller. Once the optimally aligned positionsfor the first and last laser chips at both ends of the bar aredetermined and saved in memory, then pre-aligned positions of all theother laser chips on the bar are calculated by the controller and storedin memory. The orientation of the laser bar may be adjusted based on theoptimum positions determined for the first and the last chips. With thisinformation system is ready to characterize each individual laser chipon the laser bar.

The pre-alignment and alignment of the system, e.g. the locating of thefirst and last laser chips on the laser bar and the final alignment ofthe PS module, may all be performed automatically. Alternatively, inorder to speed up the initialization process, the pre-alignment may bemanually performed by positioning stage by manually pre-aligning thefist and last laser chips, C₁ and C_(n), with the EC module, with thecontroller performing the final precise pre-alignment of the fist andlast laser chips with the EC module and storing their positions inmemory.

With the positions of each of the laser chips determined and stored inmemory, the system is now ready to characterize each individual laserchip on the laser bar. Testing and characterization of each laser chipis performed as follows. The controller translates the positioning stage120 to the aligned positions for the first laser chip C₁ in order toalign the first laser ship with the EC module. The controller then movesthe electronic probe 130 into contact with the first laser chip C₁ onthe laser bar 1. The controller then operates the first laser chip C₁ ina pulsed mode to activate the first laser chip C₁. The controller mayagain perform an automated final alignment check, which is the sameprocess as described in the automated pre-alignment procedure, toprecisely align the first laser chip with the EC module. The controllerthen rotates the rotary stage 230 to orientate the diffraction grating210 at the minimum grating angle Θ₁, for example a grating angle of42.45 degree for lasing at 4.500 um and with 300 grooves/mm grating, andoperates the first laser chip C₁ in a pulsed mode.

First, the current supplied to the laser chip is ramped up to find thethreshold current required for lasing. When an output beam is detectedusing the light output power reading (from the lock-in amplifier), thenthe supplied current is stored in memory as the threshold current forthe first laser chip C₁. The optical signal of the output beam B2 isdetected in the laser analyzer 140 and a spectrometer is demodulated bya lock-in amplifier 170. The wavelength of the output beam B2 is thendetermined by the laser analyzer and recorded in memory as thewavelength of the first laser chip at the minimum grating angle Θ₁. Thediffraction grating is then incrementally rotated to the second gratingangle Θ₂ and the threshold current and the wavelength of the output beamfor the first laser chip C₁ at the second grating angle Θ₂ is determinedin the same manner and stored in memory. This process is repeated foreach diffraction grating angle from Θ₁ to Θn, with the threshold currentand the wavelength of the first laser chip at each grating angles Θ₁ toΘn being stored in memory.

Once the first laser chip has been fully characterized, then theelectronic probe 130 is withdrawn from the first laser chip C₁ and thecontroller moves the positioning state to pre-align the second laserchip C₂ with the EC module. The electronic probe is then moved intocontact with the second laser chip C2, and the second laser chip isauto-aligned and characterized in the same manner as the previouslydescribed in relation to the first laser chip and the threshold currentand the wavelength of the second laser chip at each of the gratingangles is stored in memory. The alignment and characterization processesare repeated for each laser chip C₁ to Cn, and the threshold current andthe wavelength for each laser chip at each of the grating angles isstored in memory.

FIG. 11 is an exemplary plot of the threshold currents (illustrated bythe dots and the right-side vertical axis) and intensity in arbitraryunits (illustrated by the vertical bars and the left-side vertical axis)detected for an individual laser chip are plotted against wavelength orwavenumber (horizontal axis). As illustrated in FIG. 12, the thresholdcurrents for the characterized laser chips may be plotted againstgrating angle or wavelength and connected to produce a family ofthreshold current-wavelength curves. FIG. 12 shows only selected curvesfor lasers with the same waveguide parameters. The constant thresholdcurrent observed outside the tuning range indicates insufficientfeedback for external cavity mode lasing. It may be desirable to lase atFabry Perot (FP) modes, because the FP modes are at the center of thegain curve where the gain is the highest and therefore provide betterfeedback. The program may save all the data in four different files. Onefile for storing all measurement parameters and spectral data includingall spectra within the tuning range for each laser chip. A second filefor storing data on laser threshold vs. grating angle for each laser. Athird file for all LI curves for each laser chip. A fourth file forstoring the alignment positions for each laser chip. The data may beanalyzed and manipulated on Microsoft Excel or similar software forfuture date processing.

FIG. 13 illustrates the wavelength tuning spectra as characterized forselected laser chips on the laser bar having the same stripe width fromseven 4.6 μm quantum cascade lasers within a single laser bar. The laserbar may be, for example, a bar of EC-QCL lasers consisting of 40 lasersthat have a stripe width that periodically varies from one chip to thenext through a select range of strip widths for testing and evaluationof different stripe widths. The tunability of each laser chip can becharacterized using all the tuning spectra or the threshold currentchanges against the grating angle. For example, the tuning spectra orthe threshold current changes may be plotted as described herein.

As shown in FIG. 11, each lasing spectrum at each vertical barcorresponds to a single mode lasing at different grating angle Θ₁ toΘ_(n). The lasing wavelength shifts (or is tuned) when the gratingmirror rotates. The tunability of a laser chip is the widest wavelengthrange Θ in which the laser can lase within single mode, which can bedetermined from the difference of the wavelength (or wave number) of thelongest lasing wavelength and the shortest lasing wavelength on thehorizontal axis of the plot in FIG. 11. For example, in FIG. 11, thetunability of the characterized laser chip #28 is about 120 cm⁻¹ in wavenumber (from about 2900 cm⁻¹ to about 2780 cm⁻¹). The plots of FIG. 13show that the tunability of laser #13 is larger than that of laser #33.When the laser lases at external cavity mode (usually single mode), thelasing threshold is smaller than the threshold of lasing at Fabry-Perot(FP) modes.

As shown in FIG. 11, each black dot is corresponding to the lasingthreshold at each wavelength (or at different grating angle). There aresome black dots without spectra at the same x-coordinate (wave number).This means that the laser lases at F-P mode, instead of the externalcavity mode, at this wave number/grating angle, which is why that thosedots are at the same level. The tuning range of the laser is within therange where the threshold current is less than the threshold current atF-P mode. Therefore, by looking at the TI curves of threshold current(against the gating angle or wave number/wavelength); one can determinethe tunability of each characterized laser chip. For example, from FIG.12, one can know that the tunabilities of laser chips #35 and #25 arelarger than that of laser chips #20, #10, #5, which have narrower dip inthe curve than do laser chips #35 And #25. The smallest tuning rangebelongs to laser chip #15, which has a plot in FIG. 12 with thenarrowest dip having a width of about 93 cm⁻¹ as illustrated by thearrow 15. Whereas the largest tuning range belongs to laser chip #35with the widest dip having a width of about 159 cm⁻¹ as illustrated bythe arrow 35. With this information, one can pre-selected good laserchips for EC-QCL without going through processes such as dicing,sub-mounting, wire-bonding and, test on each device. Since the tuningrange is determined also by the quality of AR coating on each laserfacet, the performance uniformity, including AR coating uniformity,within the bar may also be examined at the same time.

Pre-selection of good laser chips for EC-QCL is provided by the systemdescribed herein without having to undergo processes such as dicing,sub-mounting, wire-bonding and, test on each device. Since the tuningrange is determined also by the quality of AR coating on each laserfacet, the performance uniformity, including AR coating uniformity,within the bar may also be examined at the same time using a system asdescribed herein.

With a system as described herein, if the external cavity is off-alignedon purpose, all the laser chips will lase at F-P mode. One can thenperform a basic function of the system, measure all the lasingwavelength of every laser chip. Since the procedure is automatic, onecan scan all laser bars from one wafer. Therefore one can map the lasingwavelength within a wafer. It will provide useful information feedbackto the active gain region designers, growers or for improvement of thewafer fabrication process.

The present disclosure provides an automatic external cavity (EC) laserchip testing and characterization system and process that is capable ofperforming auto-coupling of multiple laser chips on a laser bar to anexternal cavity, as well as automatically performing fullcharacterization of the threshold current, emission spectrum andelectrical parameters of the EC-QCL chips at different grating angles.The system described herein provides a direct and efficient way ofselecting the best gain chips in EC configurations without the need fordicing the laser bar into chips and custom mounting for EC-QCLoperation. As a result, the system described herein greatly reduces thelabor cost required to select good QC chips for EC QCL system, because:(1) the test is preformed on a laser bar, so labor intensive processessuch as dicing, sub-mounting, and wire-bonding are avoided before thetest, and (2) the system is fully automated, so that no extra labor isneeded once the system is set up to test all chips in similar laserbars. Various embodiments hereof will be understood in view of thefollowing example.

EXAMPLE

A system as described herein was used to test several different QCL gainchips developed and fabricated by Corning Inc. The total time requiredfor full characterization of the 40 lasers on a single bar wasapproximately 8 hours and included EC alignment, as well as spectral andelectrical measurements. The total measurement time was primarilylimited by the time required for the laser analyzer spectrometer and theLI curve recording.

The system was used for characterization of short wavelength QCL gainstructures with a center wavelength of 3.5 μm. The system was used totest four different QCL bars fabricated by Corning Inc.

Based on a family of LI curves collected by the system, the detectedthreshold current was plotted against the tested grating angle for alllasing chips coupled to the EC as shown in FIG. 12. FIG. 12 shows onlyselected curves for laser chips #5, #10, #15, #20, #25, #35 having thesame waveguide parameters, which allow evaluating the processhomogeneity across the wafer and quality of facet coatings. The constantthreshold current observed on both sides of the tuning range indicatesinsufficient feedback for an EC mode lasing, so that the lasing occursat the center of the gain curve as Fabry Perot modes. The entire rangeof threshold currents that are lower than the flat sections at both endsindicates the EC tuning range for each chip. FIG. 11 shows spectral andelectrical characterization data for a selected chip. The comparison ofthe threshold curve with the spectral data collected at each gratingposition confirms the relationship between the reduced threshold regionand the EC mode tuning range.

The EC laser testing and characterization system described herein iscapable of automatically coupling of each EC-QCL, or other type of EClaser chips, on a laser bar into the external cavity and automaticallymeasuring the threshold current, tuning spectrum and LI curves atdifferent grating angle of each individual laser chip, all without aneed for any manual adjustments by human operators. The total automatictesting time for a bar (40 lasers) may be about 8 hours with spectra andLI curves measurements, and may be about 4 hours with only spectralmeasurement. It is expected that the processing time can be improvedwith the use of faster equipment than has been described herein, such asa wavemeter from Bristol Instrument.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the invention. For example, the vacuum chuck isdescribed and illustrated herein as being mounted on the positioningstage for translational movement in multiple axis relative to the ECmodule. It will be appreciated that in all of the embodiments herein theEC module may be mounted on the positioning stage for translationalmovement in multiple axis relative to the chuck, and the chuck may bestationary.

1. A system for characterizing a plurality of external cavitysemiconductor laser chips on a semiconductor bar separated from asemiconductor wafer, the system comprising: (A) an external cavitymodule (an EC module) comprising: (1) a rotary stage having a rotaryaxis; (2) a diffraction grating mounted on the rotary stage; and (3) asteering mirror mounted on the rotary stage with a surface of thesteering mirror oriented perpendicular to a surface of the diffractiongrating; (B) a chuck for holding a bar of laser chips (a laser bar),with the EC module being located adjacent to the chuck so that a beam oflight emitted from a laser chip in a laser bar held on the chuckintersects the diffraction grating, a first order diffraction beam isreflected by the diffraction grating back into the laser chip to causethe laser chip to lase, and a laser beam emitted from the laser chip isreflected off the diffraction grating and the steering mirror as anoutput beam; (C) a computer controlled multiple axes positioning stagehaving one of the chuck or the EC module mounted on the positioningstage for translation of said one of the chuck or the EC module alongmultiple axis; and (D) a computer based controller for: (1)automatically moving the positioning stage and aligning a laser chip ina laser bar mounted on the chuck with the EC module; and (2) operatingthe aligned laser chip to emit a laser beam.
 2. A system as in claim 1,further comprising: (E) a laser analyzer positioned opposite thesteering mirror to receive and characterize the output beam; and whereinthe computer based controller receives signals from the laser analyzerand characterizes the output beam
 3. A system as in claim 1, wherein asurface of the diffraction grating facing the chuck has a plurality ofparallel diffraction grooves formed therein, and the diffraction gratingis mounted on the rotary stage with the diffraction grooves orientedparallel to the rotary axis.
 4. A system as in claim 1, wherein an axisdefined by an intersection of a plane defined by the surface of thediffraction grating and a plane defined by the surface of the steeringmirror coincides with the rotary axis.
 5. A system as in claim 1,further comprising a collimating lens having a lens axis, thecollimating lens being located between the chuck and the diffractiongrating such that a light beam emitted by a chip in a laser bar held onthe chuck is collimated into a collimated beam that intersects thesurface of the diffraction grating, and the collimated lens is orientedso that the lens axis is normal to the rotary axis
 6. A system as inclaim 1, wherein the chuck is formed of a conductive material.
 7. Asystem as in claim 6, further comprising a temperature control unit thatcontrol/maintain the temperature of the chuck.
 8. A system as in claim6, further comprising an electronic probe that is moveably mountedrelative to the chuck for movement under control of the controller intoand out of electrical contact with the top a laser chip to be forsupplying current to the chip, the chip being grounded through theconductive chuck.
 9. A system as in claim 1, further comprising anelectronic probe that is moveably mounted on the positioning stage formovement under control of the controller into and out of electricalcontact with the top a laser chip on the laser bar to be tested beingheld on the chuck for supplying current to the laser chip.
 10. A systemaccording to claim 9, further comprising a probe manipulator mounted onthe positioning stage and controlled by the controller, wherein theelectronic probe is mounted on the probe manipulator for movement in atleast one axis of movement of the probe into and out of engagement withindividual laser chips in a laser bar held in the chuck.
 11. A systemaccording to claim 1, wherein the positioning stage is a precisionsix-axis translation stage.
 12. A system according to claim 2, whereinthe controller further automatically rotates the rotary stage to rotatethe diffraction grating through a range of grating angles and receivessignals from the laser analyzer characterizing the output beam at selectgrating angles.
 13. A process for automatically characterizing aplurality of external cavity semiconductor laser chips on asemiconductor bar separated from a semiconductor wafer (a laser bar),the process comprising: (B) providing a diffraction unit formed of adiffraction grating and a steering mirror mounted oriented perpendicularto a surface of the diffraction grating; (C) automatically positioning alaser bar in alignment with the diffraction grating unit, such that abeam of light emitted from a select laser chip in a laser bar held on achuck intersects the diffraction grating, a first order diffraction beamis reflected by the diffraction grating back into the select laser chipto cause the select laser chip to laser, and a laser beam emitted fromthe select laser chip is reflected off the diffraction grating and thesteering mirror, such that a stable output beam is reflected off thesteering mirror as an output beam; (D) supplying current to the selectlaser chip causing the select laser chip to lase; and (E) characterizingthe output beam.
 14. A process as in claim 12, further comprising thesteps of: (G) supplying an increasing current to the select laser chipuntil the select laser chip lases; and (H) identifying the current atwhich the select laser chip lases as the threshold current of the selectlaser chip.
 15. A process as in claim 13, further comprising the stepsof: (I) automatically rotating the diffraction unit to rotate thediffraction grating through a range of grating angles; and (J)determining the wavelength of the output beam at select grating angles.16. A process as in claim 15, wherein: step (I) comprises incrementallyrotating the diffraction unit to incrementally rotate the diffractionglazing mirror through a select plurality of diffraction angles Θ1through Θ_(n); step (J) comprises, at each of the diffraction angles Θ1through Θ_(n): (1) supplying a current to the select laser chip to causethe select laser chip to lase; and (2) determining the wavelength of theoutput beam with a laser analyzer at each diffraction angle.
 17. Aprocess as in claim 16, wherein after step (J), further comprising thesteps of: (K) moving the laser bar relative the diffraction unit so thata next select on of the laser chips on the laser bar is aligned with thediffraction unit, and performing steps (I) and (J) for the next selectlaser chip; and (L) repeating step (K) until all of the laser chips onthe laser bar have been characterized.
 18. A process as in claim 16,wherein step (J) further comprises, at each of the diffraction angles Θ1through Θ_(n): (3) supplying an increasing current to the select laserchip until the select laser chip lases; and (4) identifying the currentat which each of the select laser chip lases at each of the glazingangles for each of the laser chips.
 19. A process as in claim 13,wherein step (C) is performed by mounting the laser bar in a chuck thatis mounted on a computer controlled translation stage.
 20. A process asin claim 15, wherein in step (I) the diffraction unit is rotated about arotary axis, and the diffraction unit is pre-aligned by orienting thediffraction grating on the diffraction unit such that a plurality ofparallel diffraction grooves formed in the surface of the diffractiongrating are oriented parallel to the rotary axis.
 21. A process as inclaim 20, wherein the diffraction unit is pre-aligned by orienting anaxis defined by an intersection of a plane defined by the surface of thediffraction grating and a plane defined by the surface of the steeringmirror coincides with the rotary axis.
 22. A process as in claim 20,further comprising a collimating lens having a lens axis, thecollimating lens being located between the chuck and the diffractiongrating such that a light beam emitted by a chip in a laser bar held onthe chuck is collimated into a collimated beam that intersects thesurface of the diffraction grating; and wherein the diffraction unit ispre-aligned with the positioning stage such that a select laser chip inthe laser is aligned with the collimating lens and the EC module.
 23. Aprocess as in claim 13, wherein step (C) comprises: roughly pre-aligninga select laser chip with the diffraction grating unit; automaticallytranslating one of the chuck or the diffraction grating unithorizontally and vertically within a specific range of motion;monitoring the power of the output beam and identifying the location ofthe positioning stage at which the power of the output beam is thegreatest as the aligned position for the select laser chip.